1. Field of the Invention
The present invention relates to an inexpensive, continuous process for preparing dialkyl succinates in particularly high yields from dialkyl maleates, in which only very small amounts of the 1,4-butanediol usually produced in the hydrogenation of maleic esters and no monocarboxylic and no hydroxycarboxylic acids having &lt;4 carbon atoms are formed.
Dialkyl sbuccinates are important, readily biodegradable solvents for surface coatings and plasticizers for thermoplastic polyesters having particular mechanical and chemical properties.
2. Description of the Related Art
It is known that dialkyl succinates can be prepared by esterification of succinic acid or succinic anhydride with the corresponding monoalcohols, with acid components frequently being used as esterification catalysts and high alcohol excesses being employed. It is also known that dialkyl maleates can be hydrogenated batchwise with hydrogen in a suspension process using a pulverulent Pd/Al.sub.2 O.sub.3 catalyst to give the corresponding dialkyl succinates (EP 190 424). The course of the reaction can be shown by the following reaction scheme: ##STR1##
Here, R.sub.1 and R.sub.2 can be identical or different n- or iso-alkyl radicals having from 1 to 12 carbon atoms or cyclic alkyl radicals having from 3 to 6 carbon atoms.
Batchwise processes have the disadvantage that their capacity relative to the reaction volume is very small and thus large reaction apparatuses and storage tanks are needed. Energy consumption and personnel requirements are relatively high. Continuous powder catalyst processes which employ a plurality of hydrogenation reactors connected in a cascade avoid some of these disadvantages.
However, it is still necessary for the pulverulent catalysts to be repeatedly and precisely metered in, pumped around and quantitatively filtered off from the reaction product. The catalyst slurry pumps are subject to high mechanical wear. The quantitative removal of the pulverulent catalysts from the reaction product is costly. Furthermore, there is a great danger of the catalyst activity being reduced relatively quickly by the additional operations, so that high catalyst consumptions have to be expected. It is therefore advantageous to allow the reaction to proceed over fixed-bed catalysts. Such catalysts have to have a high activity which must not decrease over a prolonged period of time because frequent catalyst replacements in fixed-bed reactions are likewise costly. In addition, there is always a desire to achieve particularly high catalyst conversions (g of dialkyt maleate/l of catalyst * h).